1. Field of the Invention
This invention relates to banded radial run-flat tires and, more particularly, to banded tires having a dual-modulus of stiffness band for enhancing the performance of the tire in the deflated or partially inflated condition.
2. Background of the Invention
An area of tire technology which has engaged investigators over the years is the run-flat pneumatic tire concept. A run-flat tire is one designed to support a vehicle for operation even if the tire has partially or totally lost its inflation pressure. The advantages of such a tire in safety, convenience, and cost are obvious. In addition, the elimination of the requirement for a spare wheel and tire results in a weight and space savings for the vehicle itself that achieves a long-sought goal in the industry. A recent successful development in the art of run-flat tires is the band-reinforced radial tire invented by one of the inventors in the present application, which banded tire is the subject of U.S. Pat. No. 4,111,249, assigned to the assignee of the present invention. The types of run-flat tires with which the dual modulus band of the subject invention can be used are disclosed in that Markow patent, U.S. Pat. No. 4,111,249, which is hereby incorporated herein by reference.
The banded run-flat tire is a pneumatic radial tire having a casing with a crown and sidewalls. The sidewalls extend radially inward from the crown on either side to annular beads which, in a conventional way, are used to mount the tire in a sealed relationship on the rim of a wheel. In the design, the band element is incorporated circumferentially in the crown region of the tire radially inwardly from the tread thereof. Any suitable high-strength material can be used for the band, preferably the band is a thin structural ring fabricated from a fiber/epoxy composite. Radial tires, as is well known, have a multiplicity of closely spaced radial reinforcing cords or wires in the sidewalls. In run-flat tires such as those disclosed in the above-referenced patent to Markow, the radial cords or wires function as spoke-like reinforcing elements to stabilize the circumferentially extending band. When the tire is deflated, the radial spoke-like elements and the band stabilized thereby form a load-sustaining structure analogous to an elastic arch. In banded tire designs, the band receives vertical, drag, and side loads from the road or ground surface. These loads are carried in compression and bending, with the spoke-like radial elements acting as tension members to support the axle. A further important function of the closely spaced radial elements is to stabilize the band against buckling.
In the interests of maximum efficiency, when operating with the tire normally inflated, the band should not interact adversely with the tire. A tire and its band in operation maintain a substantially cylindrical shape except for a flattening at the footprint or the sector of the tire in contact with the surface. In the inflated condition, the tire has a relatively small footprint where the tire and the band flattens in contact with the surface and there is a gradual bend in the transition zone at the leading and trailing edges of the footprint between the flattened portion and the normally cylindrical portion of the tire. It will be seen that a particular segment of the band (and tire) will be subject, therefore, to a cycle of bending twice during each revolution of the tire as that segment passes through the transition zones on either side of the footprint. It is also in the footprint area that most of the unpredictable transient loads caused by road surface anomalies occur.
In normal inflated operation, because the band undergoes only limited bending in the transition zone between the normally cylindrical portion and the relatively limited flattened portion, only limited tensile stresses are induced in inner surface (ID) of the band. These stresses thus are primarily in one direction only and are induced once during every revolution of the tire. Under these conditions, the band is designed to withstand about 50-million cycles of bending. Run-flat operation, in addition to ground contact stresses, induces significant bending stresses in the opposite direction as well, in that the band undergoes stresses in compression as well as in tension. Thus, in the deflated condition, fully reversed bending stresses occur twice during each tire revolution as any given band segment passes through the forward and aft transition zones of severe curvature. Here design life is in the order of about 175-thousand cycles.
In an optimum design for the band in a banded tire, stresses are kept within a calculated allowable stress to achieve an acceptable fatigue life for both normal and run-flat operation. A sufficient fatigue margin to accommodate high transient ground contact loads, which might be applied at the band edges, has to be incorporated into the design. In addition, the band has to be sufficiently stiff for it to assume its proper proportion of structural support when the tire is running deflated. It has been found that such an optimum design is difficult to achieve at reasonable cost with a single modulus of stiffness band material.
3. Description Of The Prior Art
The present invention relates to a banded radial run-flat tire having a dual-modulus of stiffness band. In the prior art, banded radial run-flat tires are disclosed in the Markow patent, U.S. Pat. No. 4,111,249, referenced previously herein. Unlike the dual-modulus bands in the present invention, the bands disclosed in that Markow patent have a constant modulus providing a classic linear load/deflection relationship when the band bends in operation.
In embodiments of the present invention, the bands preferably are fabricated from fiber/resin composites which are laid up or fabricated using filament winding techniques. A dual modulus behavior is achieved in the bands in several of those embodiments by the use of techniques involving the increasing of filament tension in the later stages of the winding process. Pre-stressing filament wound structural elements such as pressure vessels and pipes by the use of means such as expandible mandrels is well known in the prior art. However, the prior art teachings of pre-stressing techniques are directed to static structures rather than to the attainment of a dual-modulus bending behavior in a dynamic flexible annular band structure which undergoes large cyclic bending deflections as an inherent aspect of its operation.
Pre-stressing techniques for the cords or plies of tires are also known in the prior art. The techniques include the use of greater tension or extensibility of various cords or plies with respect to other cords or plies in the carcass or breaker strips or belts of conventional un-banded tires. Representative examples of such teachings are disclosed in French patent of addition No. 11,179 (1910), British No. 758,914, and U.S. Pat. Nos. 2,198,586, and 2,990,870. It will be seen that these prior art patents are addressed to details of carcass ply configurations which are intended to equalize the stress distribution in operation in loaded, inflated tires. There is nothing in these prior art tire disclosures that teaches the attainment in banded radial run-flat tires of a band having a dual-modulus of bending such that the performance in the uninflated condition of those tires is significantly improved thereby.